Silver halide photothermographic imaging materials, often referred to as 'dry silver' compositions because no liquid development is necessary to produce the final image, have been known in the art for many years. These imaging materials basically comprise a light insensitive, reducible silver source, a light sensitive material which generates silver when irradiated, and a reducing agent for the silver source. The light sensitive material is generally photographic silver halide which must be in catalytic proximity to the light insensitive silver source. Catalytic proximity is an intimate physical association of these two materials so than when silver specks or nuclei are generated by the irradiation or light exposure of photothermographic silver halide, those nuclei are able to catalyze the reduction of the silver source by the reducing agent. It has been long understood that silver is a catalyst for the reduction of silver ions and the silver-generating light sensitive silver halide catalyst progenitor may be placed into catalytic proximity with the silver source in a number of different fashions, such as partial metathesis of the silver source with a halogen-containing source as disclosed in U.S. Pat. No. 3,457,075, coprecipitation of the silver halide and silver source material as disclosed in U.S. Pat. No. 3,839,049, and any other method which intimately associates the silver halide and the silver source.
The silver source used in this area of technology is a material which contains silver ions. The earliest and still preferred source comprises silver salts of long chain carboxylic acids, usually of from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of like molecular weight have been primarily used. Salts of other organic acids or other organic materials such as silver imidazolates have been proposed, and British Pat. No. 1,110,046 discloses the use of complexes of inorganic or organic silver salts as image source materials.
In both photographic and photothermographic emulsions, exposure of the silver halide to light produces small clusters of silver atoms. The imagewise distribution of these clusters is known in the art as the latent image. This latent image generally is not visible by ordinary means and the light sensitive article must be further processed in order to produce a visual image. The visual image is produced by the catalytic reduction of silver which is in catalytic proximity to the specks of the latent image.
Many cyanine and related dyes are well known for their ability to impart spectral sensitivity to a gelatino silver halide system. The wavelength of peak sensitivity is a function of the dye's wavelength of peak light absorbance. Whilst many such dyes provide some spectral sensitisation in dry silver formulations the dye sensitisation is often very inefficient and it is not possible to translate the performance of a dye in gelatino silver halide systems to dry silver systems. The emulsion making procedures and chemical environment of dry silver systems are very harsh compared to those of gelatino silver halide systems. The presence of large surface areas of fatty acids and fatty acid salts restricts the surface deposition of sensitising dyes onto silver halide surfaces and may remove sensitising dye from the surface of the silver halide grains. The large variations in pressure, temperature, pH and solvency encountered in the preparation of dry silver formulations aggravate the problem. Thus sensitising dyes which perform well in gelatino silver halide systems are often inefficient in dry silver formulations. In general, it has been found that merocyanine dyes are superior to cyanine dyes in dry silver formulations as disclosed, for example in British Pat. No. 1 325 312 and U.S. Pat. No. 3 719 495.
Attempts to sensitise at the far red end of spectrum have produced somewhat variable results. In particular the use of cyanines to impart sensitivity in dry silver in the far red and near infra-red has given results quite inconsistent with the performance of such dyes in conventional gelatino silver halide materials. The art therefore leads towards modifying merocyanines. There are however very few merocyanines capable of absorbing at more than 750 nm and also there is uncertainty as to whether dyes which absorb will also sensitise.
The recent commercial availability of exposure sources emitting in the near infrared and in particular relatively high powered semiconductor devices emitting in this region has created a need to sensitise dry silver systems to match such exposure sources. In particular, it is necessary to match sources emitting in the wavelength range from 800 to 850 nm, which is towards the extreme end of sensitising dye art. Such materials find particular utility in laser scanning.
It has now been found that a small class of cyanine dyes possess unexpected and particularly advantageous properties of speed and stability which render the compounds particularly suitable for the use in the spectral sensitisation of dry silver systems to the near infrared.